Determination of the concentration of nitrogenous bio-organic compounds using an isotope ratio mass spectrometer operating in continuous flow mode

The quality of the determination of compound-specific isotopic content at natural abundance by gas chromatography–isotope ratio measurement–mass spectrometry (GC-irm-MS) relies on the stability of the voltage generated by the ion detector Faraday cages. The application of GC-irm-MS to the determination of δ 13 C (‰) and δ 15 N (‰) is now routine. However, for numerous applications, it is necessary to determine both the isotope content (δ 15 N) and the quantity (in micromoles) of analyte present. We now show that it is possible for nitrogen-containing compounds to measure how much analyte is present with an irm mass spectrometer linked to a GC by exploiting the integrated N 2 total ion current intensity (Vs) generated by measuring the 15 N/ 14 N isotope ratio. The method is validated over a range of concentration (2–70 mmol/L) and δ 15 N (−70 to +50‰) values for six molecules of diverse chemical nature and functionality (nortropine, norpseudotropine, nortropinone, cysteine, taurine, glutathione). It is shown that once the ion current is calibrated, the quantitative values are of a comparable quality to those obtained from GC with flame ionization detection (GC-FID). In addition, it is demonstrated that over a definable range, the δ 15 N (‰) value is independent of the quantity of analyte introduced, confirming the validity of this method.